1. Field of the Invention
The present invention relates generally to a process for the preparation of polymers including side chains containing functional groups. More specifically, the invention relates to a high solids process for the preparation of addition polymers of ethylenically unsaturated monomers, the polymer including side chains having functional groups resulting from the reaction of two predecessor functional groups.
2. Brief Summary of the Prior Art
Except for the preparation of polyolefins, almost every polymerization reaction includes monomers having one or more functional groups. Everyday examples include carboxylic acid esters and amides, present in such frequently encountered vinyl monomers as ethyl acrylate and acrylamide. While the simplest of these functional group-containing monomers are prepared commercially by more complex reactions, each can be prepared by the reaction of two predecessor compounds. For example, while ethyl acrylate can be produced on a commercial scale by reaction of acetylene and carbon monoxide in an acidified ethanol solution containing nickel carbonyl, it can also be produced by the esterification of acrylic acid with ethanol. Generally, each of the two predecessor compounds bears its own unique functional group; for example, carboxyl in the case of acrylic acid and hydroxyl in the case of ethanol. the two predecessor compounds can be viewed as each including a predecessor functional group and their reaction product can be thought of as possessing a target functional group. A number of important monomers which have relatively complex side chains are synthesized by condensation of a simple ethylenically unsaturated precursor acid with an alcohol or amine. For example, the higher alkyl esters of acrylic and methacrylic acids are manufactured commercially by direct esterification or transesterification.
In some cases, depending on the specific monomers under consideration, it has been the practice in the art to employ a monomer containing one of the predecessor functional groups (a "first functional group") in a polymerization reaction, and to subsequently add a compound containing the other predecessor functional group (a "second functional group") to a solution or dispersion containing the polymer to form the target functional group For example, U.S. Pat. No. 4,559,156 discloses copolymers of ethylenically unsaturated monocarboxylic and dicarboxylic acids, and anhydrides such as maleic anhydride, which are partially esterified with an alkoxylated (C.sub.1 -C.sub.18) alkanol.
Reacting the compound with the polymer to form the target functional group is sometimes referred to as "functionalizing" the polymer with the target functional group by those skilled in in the art.
Often when a polymer including the target functional group is to be prepared, both these synthetic pathways are at least in theory available. An example disclosing both routes is given in U.S. Pat. No. 4,524,123 (direct and indirect routes to hydroxy functional acrylic oligomers for reactive diluent use). The actual synthetic techniques selected depends on the properties of the specific system, such as the monomer reactivity ratios, the necessity of an ease of separation of the ultimate product, and the commercial availability of monomer containing the target functional group. In either case, a synthesis employing prior art techniques will include a polymerization reaction and a separate reaction between the first and second functional groups to form the target functional group.
While some vinyl monomers can be usefully polymerized in bulk for certain applications, in general vinyl addition polymerization reactions are carried out in a solvent for the monomers, in suspension or emulsion, or by some other technique which ensures that the polymeric product can be easily separated from the polymerization medium and subsequently processed. In solution polymerization, the solvent frequently plays several important roles. It solubilizes the monomers which otherwise might be incompatible, and thus expands the scope of achievable copolymerization. It may solubilize the product polymer, and serve as a carrier, diluent, or solvent for end-use products manufactured using the polymer, such as coatings compositions. The solvent may serve as a diluent for the polymerization reaction itself, making the polymerization reaction possible as a practical matter because excessive exotherms are avoided. The product polymer can be easily separated from the well chosen solvent, and the often undesired low molecular weight polymer fraction and the detritus of polymerization will remain in solution. Solution polymerization is often the technique of choice for preparing high quality product.
Despite its undeniable utility in many situations, the polymerization solvent has come to be regarded as a necessary evil, especially when it is a volatile organic compound. While its presence has always added to the raw material cost of the polymer, in recent times many useful and important solvents have been found to have undesirable effects on either the environment or occupational health or both. One alternative to use of solution polymerized polymers is reflected in the growth of water-based emulsion polymerization technology. Another approach has been the use of high solids techniques for minimizing the proportion of solvent present in the end product. In many cases, it is possible to substantially remove the organic solvent after solution polymerization by techniques such as distillation, spray drying, precipitation, and the like. Many common monomers such as acrylic esters, acrylamides, vinylidene chloride, and the like, are readily soluble in many solvents but react to form polymers which are insoluble and which precipitate from solution (precipitation polymerization).
High solids techniques often include a reactive diluent in the product formulation for post-polymerization reaction with the polymer, the diluent reacting with the polymer to form a solid in situ. For example, U.S. Pat. No. 4,672,080 discloses photocurable resin compositions prepared from urethane acrylate oligomer and including either trimethylolpropane triacrylate or tetrahydrofurfuryl-O-benzoyl-benzoate as a reactive diluent. Reactive diluents are widely used in epoxy compositions, for example U.S. Pat. No. 4,603,182 discloses an epoxy resin composition which may include divinyl benzene or diisopropenyl benzene as a reactive diluent. Similarly, European Patent Publication 119425 discloses a photopolymerizable epoxy resin composition which includes a photopolymerizable monoepoxide as reactive diluent. Alkyl glycidyl ethers are common reactive diluents in epoxy systems.
High solids acrylic coating compositions often include reactive diluents. For example, U.S. Pat. No. 4,677,168 discloses water borne high solids coating compositions employing a carbamate derivative as a reactive cosolvent or reactive diluent for the water-dispersable polymer used as a binder for the coating composition. Similiarly, U.S. Pat. No. 4,524,183 relates to high solids acrylic coating compositions formulated using a low molecular weight hydroxy-functional acrylic polymer as a reactive diluent. U.S. Pat. No. 4,515,835 discloses a high solids polymer thermosetting composition comprising a soft, low molecular weight hydroxy-functional addition polymer resin and a low molecular weight polyether reactive diluent having two or three aliphatic hydroxyl groups per molecule, the resin and the reactive diluent being dissolved in a common solvent. U.S. Pat. No. 4,477,534 relates to air-drying high solids coating compositions containing vinyl oxazoline esters as reactive diluents. U.S. Pat. No. 4,369,283 discloses high solids can coating compositions including acrylic, epoxy and aminoplast resins.
While reactive solvents and diluents are known for use in formulating high solids coating compositions and the like, they are, in most cases, mixed with the synthetic resin with which they are to react after the resin has been prepared by polymerization. One exception is the process disclosed in U.S. Pat. No. 4,144,220, in which an addition polymer is polymerized in the presence of a water insoluble polyhydric polymer, the polyhydric polymer serving as the polymerization solvent. An aminoplast resin is subsequently added in forming a coating composition curable at high temperature.
Another example of "reactive diluent" compositions is presented by resins which are partly but incompletely polymerized to form molding and impregnating resin compositions, such as B-stage epoxy resins and the like. Here the lower molecular weight portions of the incompletely polymerized resin can be thought of as a diluent for the higher molecular weight fraction. After a molding composition has been molded to the desired shape, heat is applied so that the "reactive diluent" and the high molecular weight resin react to form the solid, thermoset C-stage resin.
A related approach to high solids polymeric compositions has been the preparation of reactive oligomers, low molecular weight polymers with reactive end groups, such as reviewed in Reactive Oligomers (F. W. Harris and H. J. Spinelli, eds., American Chemical Society, Washington, D.C. 1985).
Despite the ongoing progress which has been made in reducing the level of organic solvents in high quality coating compositions by formulation of high solids coating compositions including reactive diluents, solvents continue to pose problems. Optimally, coating compositions should not emit solvents which are photochemically active or otherwise have an adverse impact on the environment or on health. In addition, organic solvents present a host of problems for the resin manufacturer. They are often flammable and toxic and must be accorded due respect in the manufacturing plant. They tend to be expensive as well. Solvents which have recently become available to reduce environmental and health concerns continue to be costly.
There is a strong need for polymerization techniques which minimize the amount of organic solvents required to produce high quality synthetic resin products. While substantial progress has been made in substituting aqueous emulsion processes for polymerization processes requiring organic solvents, the performance shown by solvent-polymerized resin products are still often superior to competitive water based products, especially with regard to water sensitivity.